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Rotational latency (sometimes called rotational delay or just latency) is the delay waiting for the rotation of the disk to bring the required disk sector under the read-write head. [22] It depends on the rotational speed of a disk (or spindle motor), measured in revolutions per minute (RPM).
Rotational latency is incurred because the desired disk sector may not be directly under the head when data transfer is requested. Average rotational latency is shown in the table, based on the statistical relation that the average latency is one-half the rotational period.
Rotational latency. This is the time it takes to correctly position the platter underneath the head so that the desired data can be accessed. (The average rotational latency is one-half of a revolution of the disk platter.) In the case of a 15,000 RPM drive, this is approximately 2 milliseconds."
HDD IOPS is proportional to RPM. When a system requests to read or write data randomly from/to a HDD, seek time and rotational latency are two HDD activities that significantly reduce HDD IOPS. Seek time is the time it takes to move the HDD head to the correct cylinder to begin to receive data. Rotational latency is the time it takes to rotate ...
Major page faults on conventional computers using hard disk drives can have a significant impact on their performance, as an average hard disk drive has an average rotational latency of 3 ms, a seek time of 5 ms, and a transfer time of 0.05 ms/page. Therefore, the total time for paging is near 8 ms (= 8,000 μs).
The behavior of disk drives provides an example of mechanical latency. Here, it is the time seek time for the actuator arm to be positioned above the appropriate track and then rotational latency for the data encoded on a platter to rotate from its current position to a position under the disk read-and-write head.
NCQ is also used in newer solid-state drives where the drive encounters latency on the host, rather than the other way around. For example, Intel 's X25-E Extreme solid-state drive uses NCQ to ensure that the drive has commands to process while the host system is busy processing CPU tasks.
This permits the drive to have more bits stored in the outside tracks compared to the inner ones. Storing more bits per track equates to achieving a higher total data capacity on the same disk area. [2] However, ZBR influences other performance characteristics of the hard disk. In the outermost tracks, data will have the highest data transfer ...